The present invention relates to a composite material comprising an aluminum strip reinforced with silicon carbide particles and a process for manufacturing said composite material. The process of the present invention avoids the use of vacuum processing steps utilized in conventional powder metallurgy techniques.
Composites comprising aluminum products reinforced with hard particles such as silicon carbide are known in the art. They have been used in a wide variety of applications including pistons for automotive engines and engine liners. Aluminum strip reinforced with a particulate such as silicon carbide, aluminum oxide, or aluminum nitride is a particularly attractive material because of highly attractive properties such as a higher elastic modulus than aluminum, a similar density to aluminum, good thermal conductivity, low thermal expansion and good tensile properties.
U.S. Pat. No. 4,623,388 to Jatkar exemplifies one type of process for producing such an aluminum composite. In this process, particles of the matrix metallic material and particles of a reinforcing material are subjected to energetic mechanical milling. The milling causes the metallic matrix material to enfold around each of the reinforcing particles while the charge being subjected to energetic milling is maintained in a powdery state. This type of milling provides a strong bond between the matrix material and the surface of the reinforcing particle. After this energetic mechanical milling is completed, the resultant powder is hot pressed in a vacuum or otherwise treated by sintering. The compressed and treated powder is then mechanically worked to increase density and provide engineering shapes for use in industry. This process is carried out at temperatures which do not cause the matrix metal to liquify (melt), wholly or partially.
U.S. Pat. No. 4,722,751 to Akechi illustrates a mechanical alloying/high energy milling process, Similar to Jatkar's, for forming a composite powder from which parts such as automotive engine components can be fabricated. In this process, heat resistant particles are first blended with a rapidly solidified aluminum alloy powder, pure metal powders or master alloy powders. The blended powders are then formed into a composite powder by a mechanical alloying technique After alloying, the composite material is subject to working such as compacting and sinter forging, cold isostatic pressing and hot forging, hot pressing or cold isostatic pressing and hot extrusion.
U.S. Pat. No. 4,661,154 to Faure exemplifies a powder metallurgy process for forming a low friction, anti-seizure product based on an aluminum alloy, a solid lubricant and at least one ceramic. In this process, a mixture of the aluminum alloy, solid lubricant and ceramic(s) is formed and then compressed in a cold condition. Thereafter, the compressed material is hot extruded in an extrusion press or sintered in the hot condition.
Commercial efforts to make a reinforced aluminum strip such as aluminum-silicon carbide have included liquid metal processes and powder metallurgy processes. The liquid metal processes such as stirring particulate into molten aluminum and casting a shape suffer from several disadvantages. For example, the volume fraction of particulate is limited to less than about 30 percent in this type of process because the mixture becomes too viscous to mix. Further, reaction rates between the liquid aluminum and the silicon carbide particulate can result in the formation of aluminum carbide which tends to degrade composite properties. From an economic standpoint, the fabrication costs of reducing the ingot to thin sheet are quite high.
Powder metallurgy processes offer a way of making much higher volume fraction composites, at least 70 percent particulate, and avoid the chemical reactivity problem. The first step of most commercial processes however involves placing the ingot in some suitable container, evacuating all atmosphere, and hot pressing or hot isostatically pressing the ingot. The principal disadvantages of this approach are that it is an expensive batch-type process and that the subsequent fabrication costs to prepare thin sheet are considerable.
It has been felt by some that aluminum-silicon carbide strip material can only be formed using a vacuum process which avoids such problems as oxidation of the aluminum powders, residual gas entrapment, and the low green strength of higher volume fraction particulates. Additionally, it was thought that the considerable deformation involved in an extrusion step was necessary to homogenize the particulate distribution and to ensure adequate bonding of matrix and particulate so that full tensile and thermal properties would be attained.